Process and apparatus for prolonging the useful lifetime of a repeatedly charged electrophotographic layer

ABSTRACT

A process and apparatus for charging of organic photoconductor layers in which the ageing effects thereon are minimized by repeatedly charging such layers to a voltage value such that the resulting secondary corona current is at most 10% of the current flowing to the layer. The saturation charging voltage is reduced and both the layer current and voltage are selectively controlled through a control electrode. Apparatus with defined physical parameters including the control electrode are fully disclosed.

United States Patent 1 Lanker 1 PROCESS AND APPARATUS FOR PROLONGING THE USEFUL LIFETIN'IE OF A REPEATEDLY CHARGED ELECTROPHOTOGRAPHIC LAYER [75] Inventor: Willi Lanker, Zumikon Switzerland [73} Assignee: Turlabor AG Zumikon. Switzerland [22] Filed: Sept. 17. 1973 [211 App]. No.: 398,134

[30} Foreign Application Priority Data Oct 16 1972 Switzerland. 15(181/72 [52] 1.1.5. Cl .w 317/262 A [51] Int. Cl. .1 HOlt 19/00 [58l Field of Search 317/262 A; 250/324-3126 [56] References Cited UNITED STATES PATENTS 1777357 1/1957 Wulkup l l r r l v 317/262 A 1 1 Aug. 19, 1975 Culhane et al 317/262 A Kamogowa et a1, .l 317/262 A Primary Examiner-J. D. Miller Assistant E.\'arnim'r-Harry E. Moose, Jr.

Almmey, Agent, or Firm-Andrew Lesniak; Milton Wolson [57] ABSTRACT A process and apparatus for charging of organic phntoconductor layers in which the ageing effects thereon are minimized by repeatedly charging such layers to a voltage value such that the resulting secondary corona current is at most 10% of the current flowing to the layer, The saturation charging voltage is reduced and both the layer current and voltage are selectively controlled through a control electrode. Apparatus with defined physical parameters including the control electrode are fully disclosed 16 Claims, I0 Drawing Figures PATENTEUAUG'I 9|915 3, 900 77 6 srm 1 [1F 5 Fig. 1

IS (a) Fig. 2

PATENTEU M181 9 I975 3. 900 7T 6 Fig. 3

Fig. 4

9 2 m \H 3 S U MW 5 l 3 w H PROCESS AND APPARATUS FOR PROLONGING THE USEFUL LIFETIME OF A REPEA'IEDLY CHARGED ELECTROPHOTOGRAPHIC LAYER BACKGROUND OF THE INVENTION The present invention refers to a process for the electric charging of a lay er, an apparatus for the carrying out of the process. and a use of the process.

The invention refers in particular to a process for charging an electrographic or clectrographic layer. It is known that gas discharges, for instance corona discharges, can produce a change in physical and chemical properties of certain substances. It has also been found that particularly organic layers. such as can be used for reproduction purposes. exhibit a pronounced ageing effect under the action of gas discharges. This ageing effect expresses itself in a faster or slower decrease in the quality of the picture upon repeated use of such an electrophotographic layer. In known reproduction processes in which such electrophotographic layers are used, the said ageing effect constitutes a considerable disadvantage. The clcctrophotographic. and particularly organic elcctrophotographic, layers must be replaced after a limited number of reproductions because of this ageing effect. Frequent replacement as well the trouble and expenses inherent therein are undesirable. The replacement of the electrophotographic layer also lends to undersircd interruptions in the readiness for use of the reproduction apparatus in question.

The object of the present invention is therefore to provide a process for the charging of layers in which the said disadvantages are avoided, and to create an apparatus for carrying out said process. The use of the process of the invention is particularly advantageous in the field of xerography.

The present invention concerns a process for the electric charging of a layer which is characterized by the fact that the layer to be charged is charged to a voltage value which is only a fraction of the new value of the saturation voltage of the said layer and further characterized by the fact that said voltage value is not higher than a voltage value V, at which secondary corona phenomena start to occur on the layer.

The present invention also relates to an apparatus for carrying out the process characterized by a holding de vice to receive the layer to be charged, a charging de vice arranged opposite this holding device or layer, said charging device having control means for adjusting the voltage obtainable on the layer by means of the charg ing device to a voltage which is at most so high that a secondary corona current which then occurs is at most 10% of the current I flowing to the layer, the control means including a control electrode with openings. and the control electrode being arranged equidistant from the layer on the side of the layer facing the charging device. the distance of the control electrode from the layer being less than four millimeters. and the size of the openings for the control electrode, at least in one part thereof, being less than l.7 millimeter.

The invention also relates to the use of the process for the repeated charging of electrographic layers.

In all figures. corresponding parts and sizes are provided with the same reference numbers. The figures have not been drawn to scale.

Before describing the illustrative examples, the problems on which they are based will be pointed out in order to provide a better understanding of the invention. These problems come from the field of electrophotography or the production of images by means of photoconductive layers. By clectrographic (exerographic) processes there are to be understood here: re production processes in which electrical properties of the layer. and particularly chargeability. resistance and conductibility. are utilized for the reproduction step. By electrophotographic processes there are to be understood here: electrographic processes in which also photoelectric properties of the layer are utilized.

See in this connection. for instance:

Dessauer and Clark: Xerography and Related Processes, Focal Press. 1965," or

Schaffcrt: Electrophotography, Focal Press, 1965."

Xerographic processes in which an electrically chargeable layer is used are known. There are also known xerographic processes in which a photoconductive layer which is electrically chargeable and selectively dischargcable by exposure is used.

One distinguishes in this connection between processes in which the said layer is only used once. for instance in the known Electrofax process, and processes in which the same layer is used repeatedly. In a process with onetime use of the layer. this layer is applied to a support. for instance a sheet of paper. First of all. a latent electrostatic charge image corresponding to the object to be copied is produced. The charge image is thereafter in known manner. either wet or dry. in order to give a visible picture. The developed picture may be fixed in known manner. for instance by the action of heat. ln electrophotographic processes with repeated use of the layer. a photoconductive layer is applied. for instance, to a cylindrical drum or to a plate or an endless belt, and this layer is used repeatedly for the production of copies. The present invention relates in particular to this last-mentioned type of reproduction processes in which a photoconductive layer is used several times.

At the present time, in order to produce such electrophotographic layers. use is made in particular of inorganic, relatively resistant. hard material. for instance selenium or an alloy of selenium with one or more other metals.

Recently. however, there have been found layers which are also suitable for electrophotographic purposes which eonsist of organic materials. alone or in combination with other substances. These organic layers, however. have a stronger ageing effect than the aforementioned inorganic layers.

Reference to such organic photoconductive materials is to be found, for instance, in:

Organic Photoconductors in Electrophotography,

LI. Grossweiner 1970, Most Associates, Marblehead, Mass, U.S.A.

By organic layers there are understood the following: electrophotographic layers in which the production of the picture depends essentially on the electrical resistance of organic material contained therein and in which this material is located also on the free surface of the layer and therefore can come into contact with gas discharges. for instance layers of:

l. Organic photoconductors, particularly photoconductive polymers such as carbazole polymers, for instance polyvinyl carbazole (PVCa) and brominated PVCa.

2. Pigment-organic binders, for instance cadmiumsulf'tde resin and phthalocyanine-resin layers. As resin, for instance epoxide resins, acrylic resins and the like.

3. Multiple layers: for instance in connection with which over a photoconductivc layer there is furthermore applied a thick organic insulating layer. for instance, of Mylar, or. for instance, a layer of PVCa on a selenium layer.

In the photocopying machines obtainable in the marketplace and using the second type of said electrophotographic processes. the development of a latent charge image is effected on the surface of the electrophotographic layer, whereupon the developed picture is transferred from the layer to a support material, for instance, to a sheet of paper. This reproduction process is known as image-transfer process.

Although in this image-transfer process the greatest part of the generally powdered developer also known as toner used for the development is removed from the layer upon the transfer of the image from the layer to the support material, traces of the developer remain adhering to the layer.

In order to avoid subsequent picture defects upon the reuse of the layer, these traces of developer must be removed as completely as possible from the layer before the latter is reused. Many different methods have already been suggested for this.

The said process step of cleaning the layer of traces of developer can be avoided when using a different re production process which is known as charge image transfer process." In this case, as in the previously mentioned process, a latent electrostatic charge image is produced on an electrophotographic layer, but, the image is transferred, prior to its development, onto another dielectric support and then subjected to the development process of known type when on said support. In this charge-image transfer process. the particles of developer do not come into contact with the clectrophotographic layer on which the original latent charge image was produced. This latent charge transfer process is described, for instance, in US. Pat. No. 2,825,814, as well as in the literature indicated above. Although it could be assumed that the charge-image transfer process should be more advantageous than the image-transfer process. the charge-image transfer process has not yet gained acceptance in practice, possibly due to the necessity of a dielectric support, instead of a normal paper support. for the production of the image. The quality of the images produced by the said two processes decreases upon repeated use of the same layer.

This impairment of the image quality is particularly pronounced in the case of the electrophotographic layers of the previously mentioned organic type.

Organic layers of the said type, for instance those of polyvinyl carbazole (PVCa), have, it is true, the advan tage of lower cost and easier manufacture than inorganic layers, but the impairment of the image quality upon repeated use of the layer (ageing effect) is much more strongly pronounced than in the case of inorganic electrophotographic layers such as, for instance. selenium layers.

The impairment of the image quality can be related to the decrease with time of the dark resistance and of the surface potential or surface-saturation voltage, also known as surface-acceptance potential. chargeacceptance voltage or saturation voltage. This is the maximum surface potential to which a given layer can be charged. Hereinafter it will he referred to as satura tion voltage." With the increasing use of a given layer, the contrast of the pictures produced with it become poorer. The contrast can also be expressed by the difference in potential which must be obtained between exposed and unexposed points of the photoelectric layer which has been electrically charged prior to exposure of the layer to a picture image.

The saturation voltage, the dark resistance, the surface conductivity and the contrast potential of the layer, as well as the local uniformity and the constancy in time thereof, are referred to here as electrophotographic properties."

One object of the present invention is to provide a method for the electric charging of layers, and particularly of layers for xerographic or electrophotographic processes, in which the aforementioned disadvantages, i.e., the ageing effects or impairment of the electrophotographic properties, or the impairments of the quality of the image produced upon longer use of the same layer, do not occur, or at least occur to a substantially less extent or substantially less rapidly. The charging of the said layers should therefore be effected in such a manner that the electrophotographic properties originally obtained with a given layer and the picture quality obtained thereby are retained as long as possible even with frequent repeated use of the layer.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described below with reference to the drawings in which:

FIG. 1 shows a charging characteristic of a new layer and of an aged coating;

FIG. 2 shows the course of the secondary corona current;

FIG. 3 shows the course of the layer voltage as a function of the number of cycles;

FIG. 4 shows current/voltage diagrams of charging devices;

FIG. 5 shows charging characteristics as a function of time;

FIG. 6 shows charging characteristics as a function of distance;

FIGS. 7, 8, and 9 diagrammatically show an apparatus usable to perform the process of the invention;

FIG. 10 diagrammatically shows a second apparatus usable to perform the process of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Since the said ageing effects occur particularly in the case of organic layers of the aforementioned type, the present invention is particularly advantageous when operating with such organic electrographic layers. In accordance with the previously known methods of charging a layer of this type, for instance when using as the charging device a high-voltage corona source of known type, the rise of the layer voltage V as a function of the current I flowing to the layer or of the charge O transferred to the layer exhibits in a new layer, for instance, the course shown in curve A of FIG. I. The layer voltage V, rises initially approximately linearly up to approximately the value V and then asymptotically approaches the value I which represents the original saturation voltage of the layer concerned in its new state. I is thus defined as the saturation voltage of a new, previously uncharged, layer.

After repeated use of the same layer, the rise in voltage still has the course. for instance, only of curve B of FIG. 1. The distance C in FIG. 1 represents a measure of the impairment which has occurred or the ageing of the corresponding layer.

A voltage level V is indicated in FIG. 1. This voltage V designates the layer voltage V at which the socalled secondary corona phenomena occur. This is expressed, for instance, by the occurrence ofa secondary corona current I also referred to as abnormal corona current in the literature.

By these secondary corona phenomena there are also understood optically perceptible punctiform discharges on the surface of a charged layer between points of different potential.

A marked relationship has now been found between the occurrence of such secondary corona phenomena and the occurrence of the ageing effects referred to hereinbelow as point defects. These point defects are visible in the copies in the form of small white points.

Referring to FIG. 2, curve D shows schematically the occurence of a secondary corona current 1 after the layer voltage V has exceeded the critical value V It can be noted from the course of curves A and B in FIG. I that the distance C which expresses the ageing becomes substantial particularly above the voltage value V From numerous tests it has now been found that this ageing of the layer and the decrease in the picture quality of the copies produced with it which results therefrom can be considerably reduced if it is seen to it that the layer is in all cases charged only up to the smallest possible fraction of V,,, preferably to a voltage value not higher than the value V and that insofar as as possible one preferably remains below the value V In this connection it has been found from numerous experiments that completely adequate picture quality can still be obtained even with such an essentially weaker charging of the layer in electrophotographic processes if one increases the saturation voltage V of the layer (for instance by an increase in the layer thickness). In this way one can maintain the operating voltage V (see FIG. 3) of the layer which is necessary for a given copying process, while at the same time reducing the fraction V /V to the preferably small value described (less than I, i.e., V V It has namely been found that with V the voltage values V and V (see FIG. 1) can also be increased and thus can be brought above a given predetermined voltage value V Current varies linearily with voltages up to value V, in magnitude and varies nonlinearily with voltage above value l/ in magnitude.

Summarizing, the following ageing effects are clearly reduced by this new method of charging electrophotographic layers:

a. The decrease of the saturation voltage V from its new value V,, with increase in the number of cycles m, in which n1 is the number of chargings followed by exposure of the layer;

)1. The voltage variations AV with time of the maximum obtainable layer voltage V after periods of rest caused by fatigue and recovery effects;

0. The local variations in potential on the layer;

d. The point defects.

In FIG. 3 a curve E shows an illustrative variation of the saturation voltage V,,-* of a given layer as a function of the number of cycles m of chargings followed in each case by discharge by exposure of the layer. A curve F shows the observed recovery efiects. By this there are meant the variations in potential AI/fl which can be noted with a temporary period of rest of the layer. In essence the curves E and F show that the voltage V to which a given layer can be charged remains approximately constant only for a relatively small number m, of cycles. i.e., within a tolerance range AV,,. Such a behavior of the layer or such a decrease in the saturation voltage V after only a relatively few cycles m does not now result in a constant quality of the pictures produced in electrophotographic processes by means of this layer even if the layer voltage in itself would still be sufficient. In order to obtain a uniform quality of picture over a large number of cycles m therefore, as a horizontal as possible a course of the saturation voltage curve as a function of m would be desired. It has now been found that in order to obtain dependable pictures by electrophotographic methods, it is not necessary at all to charge the layer used always up to the vicinity of its saturation voltage V,,-* (see FIG. 3, curves E and F). Even a substantially smaller degree of charging, for instance up to the value V (or increase of the value V,, and reduction of the fraction V 1 is entirely sufficient. This value l lies preferably in th linear portion of the curve A (FIG. I) and preferably still below the value V (see FIGS. 1 and 2), i.e., the voltage value at which secondary corona effects at which occur. If these conditions are maintained by charging of the layer at all times only up to the value V,, then the approximately horizontal curve G which starts from the ordinate value V and represents the dependence of the layer voltage on the number of cycles m will surprisingly not only ex tend to the curve E up to its intersection point H, as was to be expected, but in addition also up to the point 1. Starting with this point I, a rapid decline commences.

A tolerance range AV can be associated with the voltage value V The lower limit of this tolerance range, represented by a line K, marks the layer voltage necessary for a picture quality which is still just sufficient (this corresponds to the value V,,,, in FIGS. 5 and 6).

Recovery effects occur also upon the charging of the layer in accordance with curve G (see FIG. 3). This is indicated by a curve L. The line K intersects the curve F at a point M. A number of cycles m is associated with this point M. It would at first sight have been expected that even upon charging in accordance with line K, m would represent the maximum number of cycles then obtainable. It has now been found surprisingly, however, that the number of cycles m are actually obtainable when charging in accordance with line K lies substantially higher, namely at the point of intersection N of the line K with the curve L. The difference :m-m indicates the gain in cycles which is obtained due to the reduced charging to approximately V If it is borne in mind that FIG. 3 shows the number of cycles m on a logarithmic scale, one can note the considerable practical advantage of this new method of charging the layer to only the lowest possible fraction of the new value V,, of the saturation voltage. Numerical data will be given below with regard to the improvements in the obtainable number of cycles which have been obtained by way of example by the use of a partial charging to a specific fraction of the new value V It will also be indicated later on how such a charging can be carried out.

By numerous tests it has also been found that a compensating of the ageing effects. in other words, an increase in the number of cycles m, with constant quality of picture, can furthermore be obtained by operating the said layer by a controlled charging device described below which charges the layers with high current to as close as possible to the said voltage V, and greatly reduces the feed of current to the layer only when said value is approached.

FIG. 4 shows in curve P the course of the current 1 flowing to the layer as a function of the layer voltage V as it occurs in known charging devices. One such charging device is described by way of example in U.S. Pat. No. 2,777,957. On the other hand curve R shows the course of the current 1 which is striven for in accordance with the above remarks.

In FIG. 4, curve B shows the relationship between the layer current 1 and the layer voltage V thereby obtained in a given aged layer. From the point of intersection of this curve B which the curves P and R, it can be noted that this layer can be charged with a charging device of known type in accordance with curve P only up to the voltage V while upon charging in accordance with the method of the present invention. it can be charged up to the voltage V The value V in this case lies at the lower limit of the range of tolerance AV and the value V thus corresponds to the value V,,,,-,, in FIGS. 5 and 6. From FIG. 4 it can be seen that by the method of the invention. the charging in the region S takes place with greater current intensity I than with known charging devices. It can furthermore be noted that the current I to the layer decreases very rapidly as the layer voltage V approaches the control voltage value I}, (V lies near to V,, for instance a few percent below it) when the layer is charged in accordance with the method ofthe present invention, but that see region U in known charging devices. the layer in its new state can take up an even higher voltage. theoretically up to the value V,.

In FIG. 4 there is also entered the voltage value V of a control electrode. the importance of which will be explained later on, The voltage V,- is above said voltage value V by an amount AV In elcctrophotographic reproduction apparatus. a rapid forming of the picture is desirable; for instance a copy of an original should be produced within a few seconds. Since this production of the picture requires a number of processes such as charging of a layer, exposure of the layer in accordance with the picture, de-

velopment of a latent charge image produced thereby and possibly fixing of the developed picture, it can be seen that for each of these processes only a very short period of time. for instance only 1 second. is available. Accordingly. it is also important that the electrophotographic layer provided can be charged in a very short period of time T to a predetermined voltage value. for instance V, (see FIG. 5). In the said copy machines, the layer to be charged is, for instance. borne on an endless belt which is moved past a charging device during the said period of time T. During the period of time T, the layer moves over a path S which corresponds to the length of the charging device.

The charging process must now be completed within the period of time T, or during the movement over the path S In FIG. 5, curve W shows the rise of the layer voltage V during the charging time T for a new layer when (ill using a known charging device. Curve X shows the same relationship for an aged layer.

As compared with this, curve Y shows the rise of the layer voltage V during the charging time T for a new layer, but when using a charging device in accordance with the present invention. The curve Z shows the same relationship for an aged layer. The construction of this new charging device will be described later on. It can be noted from FIG. 5 that the lower limit value of the tolerance range AV,, i.e. the minimum layer voltage V,,,,',,, for a dependable picture quality in the case of a new layer is reached with the known charging device at time 1 in accordance with curve W. During the later course of the time interval T, the layer voltage then exceeds the tolerance range AV fixed for good picture quality, which is undesired. In contradistinction to this, a known charging device, as can be noted from curve X, in the case ofa layer has been aged somewhat, does not permit any charging to V within the period of time T which is available.

In case of strong ageing, the value V,,,,,, could not be obtained at all, no matter how long the charging time was. In accordance with FIG. 5, the difference between the potential of the new layer which is reached and that of the aged layer as AV. this difference AV being substantially greater than the tolerance range AV With known charging devices, it is possible in principle, it is true, either to charge a new layer or an aged layer to close to the value V but both can never be done with the same charging devices; the difference in potential AV in each case remains greater than the tolerance range AV In other words, known charging devices can supply either a sufficiently high current I or provide a good control of the current I but cannot do both at the same time. as would be necessary for the carrying out of the present process. This will be explained in further detail below on basis ofthc examples.

With a charging device in accordance with the pres ent invention. the value V,,,,,, with a new layer is reached in accordance with curve Y already at the time I" and nevertheless the tolerance range AV, is not exceeded even by the end of the time interval T. With a layer aged to the same extent as for curve X on the other hand, the new charging device just still reaches the value V,,,,-,, by the end of the time interval T.

The difference in potential AV between new and aged layers is here therefore substantially smaller than with the known charging device and which is decisive feature the difference AV is smaller than the tolerance range AV,.

The method thus makes possible only a reduction of the ageing of the layer as a result of the charging to only a fraction of the new value of the saturation voltage (see FIG. 3), but also, at the same time, a compensation of the resultant ageing by a controlled charging process, which has been described with reference to FIG. 4. This compensation is obtained essentially in the manner that the layer is first of all charged with strong layer current I, very rapidly to close to the value V whereupon a further charging to approximately V, takes place until the end of the time interval T with current which decreases strongly as the charging voltage approaches the value of the voltage V FIG. 6 shows the rise in voltage as a function of the path S over which the layer moves as it travels through a charging device. In this connection S designates the path distance through which a layer travels during the time T (cf. FIG. 5 l. The curves Y and Z apply for new when used in aged layers and the new charging device in accordance with the invention. In connection. FIG. 6 in addition refers to another advantage of the present invention which will be further explained later on. By this further advantage of the invention. the rise of the layer voltage as a function of the path distance is imparted the even more favorable course shown in curve Y" for a new layer and the course shown in curve Z" for an aged layer. The substantial shortening of the paths S,-.. as compared with S, and S as compared with S and the reduction of the potential difference AV" compared with AV. shows the improvement obtained by using the invention.

Before going into a description of the apparatus for the carrying out of the process. a summary will be given of advantageous variants of the process.

It is advantageous to associate with the said voltage value V a given tolerance range AV, which corresponds to the useful potential range for good picture quality. This tolerance range AV is generally advantageously selected at most at 30%, referred to V,. The fraction of the new value V., to which the layer is charged is preferably selected at most 50% thereof. The value V, to which the layer is charged is preferably so selected that it still lies in the linear portion of the charging curve of the new layer; sec curve A. FIG. 1. It is particularly advantageous not to select the value V smaller than V and in any event not substantially higher than the voltage value V at which secondary corona phenomena begin to occur on the layer. For example, V is selected at most so much above V that the secondary corona current l occurring thereby amounts to at most l7r of the current flowing to the layer.

FIG. 7 shows diagrammatically a first embodiment of an apparatus for the carrying out of said process. This first embodiment can be considered a basic embodiment of the apparatus which can be still further improved by other developments. The apparatus as a whole is designated as l. A holding device 2, preferably grounded. and consisting. for instance. of a plate or endless belt. bears the layer 3 which is to be charged and which consists. for instance. of polyvinyl carbazole. The holding device 2 and the layer 3 have a charging device associated with them. This charging device 4 has, for instance. a source 5 ofd.c. voltagehigh voltage. the one terminal 6 of which is grounded, while its other terminal 7 is connected via a line 8 with a corona electrode 9. The corona electrode 9 preferably consists of thin wires, for instance of a diameter of 75a. The highvoltage source 5 feeds the corona electrode 9 a dc. voltage of. for instance, l5 kilovolts. the polarity of which is selected in accordance with the polarity desired for the layer charge.

The corona electrode 9 several electrodes can also be connected with the high voltage source S is stretched. for instance. between two side walls 10 and 10 of the charging device 4. In the case of side walls which preferably consist of insulating material. the corona electrodes 9 can be fastened directly to them.

The housing of the charging device 4 is closed by a cover made of insulating material 12. At the lower edge of the side walls [0 and 10' and/or at a front or rear wall 11 and 11' (not shown in FIG. 7) a control electrode 13 is arranged. The control electrode 13 has thin wires arranged, for instance. at a small distance apart and parallel to each other or in grid shape. the electrode 13 may consist of a plate or the like having fine perforations. The inside dimensions of the openings in the control electrode 13 is designated w. The control electrode 13 is connected via a line [4 with the one terminal 15 of a preferably. but not necessarily. variable control-voltage source 16, having the voltage V whose other terminal 17 is grounded. The polarity of the control voltage V is selected the same as that of the corona electrode.

In order that the control electrode 13 will be as flat as possible, it can be stretched on a separate frame which is then fastened to the corona walls. Between the corona electrode 9 and the cover 12 there is a distance (1,. between the corona electrode 9 and the control electrode 13 a distance d and between the corona electrode 13 and the layer 3 a distance d In the case of FIG. 7. it has been assumed that the layer 3 is moving together with its holding device 2 per pendicular to the plane of the drawing. for instance towards the rear. is indicated by an arrow 18. Since the distance d between the control electrode [3 and the layer in accordance with the invention is very small. namely at most 4 mm. and preferably however. only about 1-2 mm. it may be advantageous to provide lat eral guide elements to maintain the spacing. Such guide elements 25 can. for instance. consist of highgrade insulating material of low coefficients of friction such as Teflon and the like. In this connection it is important that the control electrode 13 be arranged as accurately as possible equidistant with a small spacing 11;, to the layer 3. and that the inside dimension n of the openings of the control electrode 13 be at least in part thereof smaller than 1.7 mm.

Only if these dimensions are satisfied are the effects intended by the present invention obtained. Superficially similar charging devices are already known (see. for instance. US. Pat. No. 2.777.957 Walkup) which. however. as a result of their structure or arrangement, do not make it possible to achieve the goals achieved by the use of the apparatus described herein. With an arrangement in accordance with the present invention. upon the charging of the layer 3 in new condition as well as in aged condition. layer voltages V.- are obtaincd, both of which lie close to the desired optimum value V,, as has been described with reference to FIGS. 5 and 6. In order to achieve this, the control voltage V of the control electrode 13 or the control voltage source l6 must be selected close to the value V for instance about 5% above the value V The control volt age V is. for instance. about 800 volts.

The distance (1;, is then maintained only slightly above the value which results as breakdown distance to the layer 3 for the control voltage V and preferably less than three times as great. A value of about 1 millimeter then results. for instance, for :1 Furthermore. the inside dimension w of the openings is selected very small. preferably 0.84.2 millimeter. As a result of the small spacing (1;, and the small inside dimension w. there is obtained a sufficiently high current I to the layer with at the same time a good control thereof.

FIG. 8 shows a cross section through the apparatus 1 extending at right angle to the showing of FIG. 7. From it there can be noted the spacing :1 between the individual corona wires 9 and the edge spacing (i to the front wall 11 and the rear wall 11' respectively of the apparatus 1 (for the sake of completeness, the highvoltage source S and the control-voltage source I6 together with their connections have been shown also in FIG. 8).

In FIGS. 8 and 9 the currents I,,, I and I flowing in the charging device have been shown schematically. In this connection I,, and I designate the currents which flow from the corona electrode 9 to the control electrode l3 and the cover 12 resepctively, and I, is the current which flows from the control electrode 13 to the layer 3.

FIG. 9 shows in a detailed view the distance a between the wires of the control electrode 13, the thicknessfof these wires, the inside dimension nof the resulting openings and the distance (1;, to the layer 3.

In order now to be able to obtain a good charging characteristic such as, for instance, that shown in FIG, 5 by the curves Y and Z and in FIG. 6 by the curves Y" and Z, it is necessary that the charging device give off a current characteristic l (V represented by the curve R in FIG. 4 and therefore a sufficiently high and accurately controlled current I, to the layer. This requires, in addition to the control discussed above by the control electrode 13, also a high current I to the control electrode, which can be obtained with a highcurrent corona. By high-current corona there is understood a muIti-wire corona charge device which has the features described below in order to obtain therewith a high current I, as well as a high current density l /cm It requires on the one hand optimum spacings of the corona wires with respect to each other as well as with respect to the walls. In an apparatus having insulating walls, in accordance with FIG. 8, for instance with d l5 millimeters: d J and 41,-, L1,.

In order for a corona wire to give off the highest possible current, it must be operated with the highest possible voltage, i.e., as close as possible to the breakdown voltage. Since this current rises, however, very strongly towards the breakdown voltage, the breakdown voltage in the case of multi wire corona electrodes 9 must as far as possible be the same everywhere and for all wires so that the corona electrode can be operated in actual operation in a voltage which lies only slighly below the breakdown voltage, without local breakdowns occurrings, and so that the same high current is given off everywhere. In order to achieve this, in particular the marginal field on the outermost corona wires must be compensated. In accordance with the invention, this can be done by the use of insulating front and rear walls II and II in preferably experimentally determined, optimum spacing d (d (1,). If front and rear walls consist of conductive material, then a pairwise arrangement of the corona wires such as described in FIG. 10 has proven advantageous in order to obtain a breakdown voltage which is as uniform as possible for all wires.

With such high-current corona arrangements, an oscillating of the corona wires frequently occurs. This effect naturally favors a voltage breakdown and should therefore be comhated as much as possible. This can be done, for instance, by a correspondingly tight stretching of the wires and/or by a supporting of the wires. However, this oscillating can be prevented in particular if the total current I I,, is reduced by making the cur rent to the cover I as small as possible, with constant current I This can be obtained if, with a conductive cover 12a and a perforated conductive plate 12D as in FIG. 10, the distance d, is selected larger than the distance d for example (1, I5 to 2A or if an insulating covering is used as in FIG. 8.

Insulating walls and covers must in this connection consist of corona-resistant material, for instance of Teflon, Kapton or, with certain limitations, also Plexiglas. The reduction of the current I also has the further advantage that in this way the production of ozone by the corona is reduced.

On basis of FIG. 10, another illustrative embodiment will now be described. Here the apparatus as a whole is designated IA and is charaterized by the fact that a holding device 2A is developed as an endless belt, for instance a metalized nylon belt, which bears a flexible chargeable layer 3A, for instance of polyvinyl carbazole. The charging device 4A is characterized by the fact that its corona electrode 9A consists of pairs of wires 9A and 9A" stretched at a distance d apart, the distance 41 between two adjacent pairs being greater than d The edge distance d to the conductive walls I I and l l is selected in particular with regard to the edgelield distortion in such a manner as to result again in a breakdown voltage which is as uniform as possible. Favorable values of spacing are, for instance, for d 15 millimeters and d 12 millimeters: d 2d and (1,. z d,.

In accordance with FIG. 10, the cover 12A of the charging device 4A has a central opening 128 defined by a tubular extension 12C. A perforated plate 12D is furthermore provided.

As control electrodes there are arranged two controlelectrode sections 13A and 133 corresponding to the path lengths S, and S in FIG, 6, and at the same potential V the inside dimension w of the openings in the section, or zone, 13A being greater than of the openings in the section, or zone 133. While the originally uncharged layer 3A moves past the charging device 4A in the direction indicated by the arrow 18, it becomes charged with relatively strong, coarsely controlled current I,,- as a result of the larger inside dimension of the openings in the section 13A. The length of the section 13A is selected in such a manner that a new layer SA has almost reached the value V for instance about 90% thereof, at the point 22.

During the further passage of layer 3A underneath the region 138 having the small inside dimension openings, only a reduced, finely controlled current I,- still flows to the layer and charges it very accurately up to the value V The charging to the desired voltage value V is thus effected in two phases. A further improvement can possibly be obtained if the first controleleetrode section 13A has, for instance, a control voltage which is 5l0% less than the second section 138.

In order to maintain the distance (1 which is controlling for good control action as constant as possible, layer 3A and control electrode 13A, 133 must as far as possible be flat and parallel to each other, i.e., variations in the distance d should be kept as small as possible. This requirement is not easy to satisfy in particular in the case of a flexible layer and holding device (which tend to curl) such as used in FIG. 10. Suitable guide means must be used. For instance, in this case the holding device 2A coming from the left is pulled below a guide roller 19 and to the right over a cylindrically curved guide piece 21.

The guide roller is fastened by means of a supporting arm 20 in exact position with respect to the charging device 4A and its control electrode 13A. I38. By this guiding of the holding device 2A (the nylon belt), a substantially parallel relation of layer 3A with respect to the control electrode 13A, 13B is obtained over the entire length L of the portion of the layer 3A beneath the electrode 13A. 138. Additional longitudinal guide elements 25 (not shown in FIG. such as i dicated in FIG. 7 result in further improvement. Even better constancy of the distance d can be obtained by guiding a flexible layer and holding device under slight tension over a cylindrically curved, fixed guide base which extends equidistant to a control electrode which is curved in the same manner. The radius of curvature can be very great here and amount to several meters.

Over the tubular extension I2C there is placed a hose 23 which is connected with a suction device (not shown) of known type. At least during the operation of the corona electrode, air is thereby drawn out of the charging device 4A. This results in a flow in the direction indicated by the arrows 24, directed substantially from the layer 3A to the corona electrode. In this way the chemically corrosive products of the corona discharge which otherwise substantially increase ageing, such as for instance ozone. are kept substantially away from the layer 3 or 3A to be charged. By this measure also, the possible number of cycles is substantially increased, for example an increase in the life by a factor of 10 was found.

With devices in accordance with the present invention such as described. for instance with reference to FIG. 10, the charge characteristics Y" and Z" see FIG. 6 were obtained. It can be seen that the value V,,,,-,, is reached upon movement over the path S,-". and

strongly aged,

standard layer slightly aged. standard layer thick layer that in its further course the charging takes place very accurately to the value V,. Even an aged layer is charged to V,,,,-,, over a relatively short path 5;", and it is further charged in its additional travel to a voltage lying within the tolerance range AV,. so that the difference in potential AV" is desirably small.

Considerable improvements with respect to the possible number of cycles and the uniformity of picture can be obtained by the method of the invention and with the apparatus of the invention, as will become evident from the examples mentioned below.

The ageing experiments described in Examples I and 2 were carried out in the following manner: organic electrophotographic polyvinyl carbazole layers were charged times per minute by corona charge devices to a given potential V, and with a given current I and then discharged by exposure. A charging current I of IOUaamp corresponds in this connection to a charge Q of 8.10 C/cm applied in one cycle to the layer.

Standard layers: polyvinyl carbazole layers sensitized with 3% tctranitrofluorenone, in a thickness of 5 pt on aluminum electrodes. New value of the saturation volt age V 900 V.

Thick layers: increase layer thickness (=18 a), V,, 2.200 V. The life is defined as the number of cycles m at which the saturation voltage V has dropped to the Standard layers: V, V,/V., l Life about strong ageing 800V 0.) high m, 200 cycles slight ageing 300V 0.3 (l m;. XX UU Cycles Thick laycrsi slight ageing 800V (1.3 (I m; l(l (ll)(l cycles The life is therefore drastically increased by reduction of the fraction V,/V,,, which can be obtained by reduction of V, or by increase of V,,. This also applies to the following example:

2nd Example (point defect reduction) After ageing with constant charging current I (in which V, decreases), the point defects were determined by producing electrophotographic copies with the aged layers under standardized conditions. The ageing effect is characterized by n. the number of points per cm occurring on the copies. Result:

n increases with the number of cycles m n increases very greatly with the intensity of the ageing. i.e.. with I and I It has been found that by slight ageing, the point defects can be reduced very strongly. particularly if the secondary corona current I =0, or V is less than V (see FIG. 2). This is also clear from the following experimental data:

for m 2500 cycles:

l V, (new) n amp 40 amp 800 V IOU points/cm rn IODOO cycles:

18 amp 0 300 V 5 points/cm 40 amp 0 800 V U points/cm Again by increasing V,,. with V unchanged, :1 strong reduction in the ageing can be obtained by means of thick layers.

3rd Example (control) Optimal control of a high current I such as represented by the curve R in FIG. 4, permits of good compensation of the ageing effects and thus the carrying out of the process of charging a layer which changes in the course of ageing to a potential V V,, which is as constant as possible. This requires on the one hand a high current I to the control electrode, which the highcurrent corona devices described supply, and furthermore the highest possible current passage i /l as well as a good control action of the control electrode. The control action (or control) can be characterized by the slope i of the current-voltage characteristic 1' (V of the control electrode at the voltage value V of the control voltage. This slope is advisedly defined as i' d (log i)/ (IV The control is further characterized by the difference in potential AV V! (FIG. 4). V is defined as the voltage for which i 0.01.

In order to obtain high current passage 1 (the ratio lS/ and good control, the distance 0' between the control electrode and the layer is of decisive importance, as well as the inside dimension w of the openings in the control electrode. This can be noted from the following comparison of experimentally determined values for known control electrodes (known from U.S. Pat. No. 2.777957) and new control electrodes in accordance with the present invention:

of an original are derived in conventional fashion, and including apparatus adapted for use in repeatedly charging and discharging the layer to and from. respec- Known d; w i( V -600V) i'( V,-,) A V Control electrodes: 6} mm 4 mm 0.4 0.05. H) "-'/V I000 V [AV 300 V) 6,} =l 4 mm I00 V New 2 (L7 0.25 4 i V Control electrodes: '1 L4 ()4 l l0 V [AV' z 70 VJ I (L5 (L3 6 Ill V lAV" 40 V] l 0.7 0.35 4 0 V l L4 0.55 l V There would he desired for instance. for high current, i '(l 2 and for good control i lll /V JAVJSU V.

lt was found that with decreasing spacing (1 i increases greatly. without the control changing substantially, while with decreasing size w: i also decreases. but the control becomes better.

Therefore (1;; must be selected as small as possible and w thereupon optimized. Only in this way can the two requirements of high current as well as good control be simultaneously satisfied, as is necessary for the carrying out of the method in accordance with the invention.

Previously known devices due to an undefined or excessively large spacing (1;; could not satisfy the two conditions simultaneously, and therefore could not simultaneously charge both new and aged layers to approximately the same value. This is expressed by the potential differences AV, AV and AV" (see FIGS. 5 and 6), which should lie within the tolerance range AV ln this example AV would be about lOO-lSO V, whereby AV (known charge device) is again much too large, but AV and AV" (for the present devices) are smaller than AV,.

What is claimed is:

1. Apparatus comprising:

a holding device receiving a layer thereon;

a charging device including means for charging the layer to a voltage value (V less than the new value (V,,) of the layers saturation voltage so that the secondary corona current (1 is at most l0% of the current (l flowing to the layer;

the charging device including a control electrode having openings therein with such openings in at least a part thereof being of a size (w) less than l.7 millimeters; and

the electrode being spaced less than 4 millimeters from the layer and being arranged equidistant therefrom on the side of the layer facing the charg ing device.

2. The apparatus for claim 1 wherein said charging device further includes means for adjusting the voltage value (V,).

3. The apparatus ofclaim I wherein the voltage value (V is not higher than that voltage value (V at which secondary corona phenomena start to occur on the layer.

4. The apparatus of claim 3 wherein said charging device further includes means for adjusting the voltage 1)- 5. In a photocopy machine comprising an electrostatically chargeable layer from which toned. fixed pictures tively. a voltage value (V less than the new value (V,,) of the layers saturation voltage, the voltage value (V being maintained in a voltage tolerance range (AV wherein sufficient image picture quality is obtainable the apparatus comprising:

a holding device for receiving the layer to be charged;

a charging device associated with the holding device and the layer, the charging device having a control means for adjusting the voltage (Vs) on the layer obtainable with the charging device to a voltage value (V less than the new value (V of the lay ers saturation voltage, the voltage value (V,) being at most so high that the secondary corona current (l which then occurs is at most 10% of the current (l flowing on the layer; and

the control means including a control electrode having openings therein with the size (w) of the openings in at least a part thereof being smaller than 1 .7 millimeters. the electrode being equidistant to the layer on the side of the layer facing the charging device. and the spacing (d of the electrode from the layer being less than four millimeters.

6. The apparatus of claim 5 wherein the voltage value (V is not higher than that voltage value (V at which secondary corona phenomena start to occur on the layer.

7. The apparatus of claim 5 wherein the voltage value (V is at most of the new value (V of the layers saturation voltage.

8. The apparatus of claim 5 wherein the tolerance range (AV,) is at most 30% of the voltage value (V,).

9. The apparatus of claim 5 wherein the voltage value (V is in a linear portion of the charge curve for a new layer.

10. The apparatus of claim 5 wherein the control electrode has a first zone in which the openings are at most 2 millimeters in size and has a second zone in which the openings are at most 1.5 millimeters in size, the area of the first zone being smaller than that of the second zone,

11. A process for restricting aging of a layer that is repeatedly charged and discharged. characterized by the fact that the voltage value (V to which the layer is charged is a. less than the new value (V,,) of the layers saturation voltage, but higher than the voltage value (V at which secondary current (l starts to flow,

h. in a voltage tolerance range (AV in which sufticient picture quality is obtainable from the layer when it is used to provide electrostatic images from which toned, fixed pictures of an original are derived in conventional fashion, and c at most so high that the secondary current (l i) which then occurs is at most 10% of the current (l flowing to the layer, whereby the useful lifetime of the layer is prolonged.

12. The process of claim ll wherein the voltage value (V is at most 50% of the new value (V,,) of the layer's saturation voltage.

13. The process of claim 11 wherein the tolerance range (AV,) is at most 30% of the voltage value (V 14. A process for restricting aging of a layer that is repeatedly charged and discharged, characterized by the fact that the voltage charge (V to which the layer is charged is a. less than the new value (V,,) of the layers saturation voltage,

b. in a voltage tolerance range (AV,) in which sufficient picture quality is obtainable from the layer when it is used to provide electrostatic images from which toned, fixed pictures of an original are derived in conventional fashion, and

c. not higher than that voltage value (V at which secondary corona phenomena start to occur on the layer,

whereby the useful lifetime of the layer is prolonged.

15. The process of claim 14 wherein the voltage value (V is at most 50% of the new value (V,,) of the laycrs saturation voltage.

16. The process of claim 14 wherein the tolerance range (AV is at most 30% of the voltage value (V 

1. Apparatus comprising: a holding device receiving a layer thereon; a charging device including means for charging the layer to a voltage value (V1) less than the new value (Vo) of the layer''s saturation voltage so that the secondary corona current (Isec) is at most 10% of the current (Is) flowing to the layer; the charging device including a control electrode having openings therein with such openings in at least a part thereof being of a size (w) less than 1.7 millimeters; and the electrode being spaced less than 4 millimeters from the layer and being arranged equidistant therefrom on the side of the layer facing the charging device.
 2. The apparatus for claim 1 wherein said charging device further includes means for adjusting the voltage value (V1).
 3. The apparatus of claim 1 wherein the voltage value (V1) is not higher than that voltage value (Vsec) at which secondary corona phenomena start to occur on the layer.
 4. The apparatus of claim 3 wherein said charging device further includes means for adjusting the voltage (V1).
 5. In a photocopy machine comprising an electrostatically chargeable layer from which toned, fixed pictures of an original are derived in conventional fashion, and including apparatus adapted for use in repeatedly charging and discharging the layer to and from, respectively, a voltage value (V1) less than the new value (Vo) of the layer''s saturation voltage, the voltage value (V1) being maintained in a voltage tolerance range ( Delta V1) wherein sufficient image picture quality is obtainable, the apparatus comprising: a holding device for receiving the layer to be charged; a charging device associated with the holding device and the layer, the charging device having a control means for adjusting the voltage (VS) on the layer obtainable with the charging device to a voltage value (V1) less than the new value (Vo) of the layer''s saturation voltage, the voltage value (V1) being at most so high that the secondary corona current (Isec) which then occurs is at most 10% of the current (IS) flowing on the layer; and the control means including a control electrode having openings therein with the size (w) of the openings in at least a part thereof being smaller than 1.7 millimeters, the electrode being equidistant to the layer on the side of the layer facing the charging device, and the spacing (d3) of the electrode from the layer being less than four millimeters.
 6. The apparatus of claim 5 wherein the voltage value (V1) is not higher than that voltage value (Vsec) at which secondary corona phenomena start to occur on the layer.
 7. The apparatus of claim 5 wherein the voltage value (V1) is at most 50% of the new value (Vo) of the layer''s saturation voltage.
 8. The apparatus of claim 5 wherein the tolerance range ( Delta V1) is at most 30% of the voltage value (V1).
 9. The apparatus of claim 5 wherein the voltage value (V1) is in a linear portion of the charge curve for a new layer.
 10. The apparatus of claim 5 wherein the control electrode has a first zone in which the openings are at most 2 millimeters in size and has a second zone in which the openings are at most 1.5 millimeters in size, the area of the first zone being smaller than that of the second zone,
 11. A process for restricting aging of a layer that is repeatedly charged and discharged, characterized by the fact that the voltage value (V1) to which the layer is charged is a. less than the new value (Vo) of the layer''s saturation voltage, but higher than the voltage value (Vsec) at which secondary current (Isec) starts to flow, b. in a voltage tolerance range ( Delta V1) in which sufficient picture quality is obtainable from the layer when it is used to provide electrostatic images from which toned, fixed pictures of an original are derived in conventional fashion, and c. at most so high that the secondary current (Isec) which then occurs is at most 10% of the current (IS) flowing to the layer, whereby the useful lifetime of the layer is prolonged.
 12. The process of claim 11 wherein the voltage value (V1) is at most 50% of the new value (Vo) of the layer''s saturation voltage.
 13. The process of claim 11 wherein the tolerance range ( Delta V1) is at most 30% of the voltage value (V1).
 14. A process for restricting aging of a layer that is repeatedly charged and discharged, characterized by the fact that the voltage charge (V1) to which the layer is charged is a. less than the new value (Vo) of the layer''s saturation voltage, b. in a voltage tolerance range ( Delta V1) in which sufficient picture quality is obtainable from the layer when it is used to provide electrostatic images from which toned, fixed pictures of an original are derived in conventional fashion, and c. not higher than that voltage value (Vsec) at which secondary corona phenomena start to occur on the layer, whereby the useful lifetime of the layer is prolonged.
 15. The process of claim 14 wherein the voltage value (V1) is at most 50% of the new value (Vo) of the layer''s saturation voltage.
 16. The process of claim 14 wherein the tolerance range ( Delta V1) is at most 30% of the voltage value (V1). 